Optical memories and optical random and pseudo-random pattern generators are important components for optical communication and computing systems such as ultra-high-speed, time-domain, multiplexing, multi-access optical networks. Such devices are useful for performing a variety of functions, including storing data packets during dock recovery, processing of headers, and data rate conversions. Also, optical memory is required for bandwidth-on-demand systems while users wait for access to the network.
Short-term optical data storage has been demonstrated in optical memories. For example, U.S. Pat. No. 4,473,270 discloses an optical circulating loop useful for a short-term optical memory. Data is loaded into the circulating loop and is preserved during multiple circulations in the loop. The data signals are readable until they are attenuated. Because there is no amplification in the loop to compensate for loss, the data signals rapidly attenuate.
U.S. Pat. Nos. 4,738,503 and 4,923,267 disclose an optical circulating loop which includes an amplifier to partially compensate for losses in the loop. The amplifier, however, must operate with a net round trip loss, otherwise noise can build to a large steady-state value. In addition, laser oscillation will occur and destroy the data pattern.
Researchers have discovered that lossless circulation in an optical circulating loop can be achieved by incorporating bistability in the circulating path. J. D. Moores, "On the Ginzburg-Landau Laser Modelocking Model with Fifth Order Saturable Absorber Term," Opt. Comm., vol. 96, pp 65-70, February 1993, H. A. Haus, E. P. Ippen, and K. Tamura, "Additive Pulse Modelocking In Fiber Lasers," IEEE J. Quant. Elec., vol 30 pp. 200-208, January 1994. Bistability introduces different round trip losses for high intensity and low intensity signals. Thus, the bistability amplifies and maintains optical pulses with higher energy and attenuates optical pulses with lower energy.
Storage time in circulating loops having lossless circulation is restricted by propagation limitations. Mechanisms which contribute to propagation imitations include the Gordon-Haus effect, Raman self-frequency shift, and third-order fiber dispersion. J. D. Moores, W. S. Wong, and H. A. Haus, "Stability and Timing Maintenance in Soliton Transmission and Storage Rings", Opt. Comm., 113, p. 153, (1994).
The Gordon-Haus effect is a noise-imparted propagation limitation which occurs when spontaneous emission noise from amplifiers shifts the frequency and thus, the velocity of an optical pulse through group velocity dispersion. These random velocity shifts result in timing errors. The timing errors limit the achievable bandwidth-transmission distance product. In optical memories, the Gordon practical storage time of the memory.
Raman self-frequency shift is another propagation imitation which occurs with short-pulse transmissions and is due to the fad that the pulse frequency shifts with propagation distance at a rate proportional to the squared pulse bandwidth and the intensity. Noise-imparted fluctuations in pulse photon number and pulse width alter the rate of Raman sell-frequency shift of a pulse, or equivalently, alter the rate at which the inverse group velocity changes with distance and result in additional timing errors. This Raman effect is a serious limitation for high-speed long-distance transmissions and long-term storage.
Third order dispersion also limits propagation and storage time. Classically, it leads to distortion of pulses, including solitons. Furthermore, noise-imparted fluctuations in pulse bandwidth result in timing jitter. Other effects which may limit propagation and storage time include electrostriction and inter-pulse interactions. P Researchers have discovered that these propagation limitations can be overcome by incorporating a stabilizing element in the circulating loop. This allows long-term storage without pulse degradation, timing jitter or photon number fluctuations. C. R. Doerr, W. S. Wong, H. A. Haus and E. P. Ippen, "Additive-Pulse Mode-locking/Limiting Storage Ring"; Opt. Lett., 19, p. 1747, (1994). Prior art stabilizing elements utilize electronic or electro-optic devices modulated by an electrical signal to control optical transmission within the circulating loop. The data rate in the circulating loop is limited by the bandwidth of the electronic or electro-optic devices. Unfortunately, the bandwidth of these devices limits the data rate in the circulating loop to around 10-20 GHz.
It is therefore a principal object of this invention to provide a circulating loop memory in which the stabilizing element is all-optical and, therefore, is not limited by the bandwidth of electronic or electro optic devices. It is another object of this invention to provide an all-optical stabilizing element that utilizes known ultrafast optical transmission nonlinearities of semiconductor amplifier devices. Such a stabilizing element allows the storage of a high-speed optical data pattern for long periods of time. It is another object of this invention to provide a monolithically integrated all-optical memory suitable for a compact optical communication system. It is another object of this invention to provide an optical pattern generator for producing high-speed optical random and pseudo-random signals.